The circuit shown in FIG. 2 is a conventionally known LED drive circuit for driving LEDs. PTL 1 is a known example of literature that discloses such a circuit.
The LED drive circuit 100 shown in FIG. 2 includes multiple LEDs 101 and a DC power supply 102 that is connected in series to the LEDs 101. The LED drive circuit 100 also includes a coil 103, a rectifying element 104, a transfer switching element 105, a current sensing resistor 106, and a control apparatus 107.
The LEDs 101 are connected in series and emit light when a forward bias is applied. Also, the DC power supply 102 is disposed so as to apply a forward bias to the LEDs 101, with one end (the negative terminal) being connected to a ground G, and the other end (the positive terminal) being connected to the anode side of the LEDs 101.
The coil 103 is connected to the cathode side of the LEDs 101, and is connected in series with the LEDs 101 and the DC power supply 102. This coil 103 can accumulate energy from current generated by operation of the DC power supply 102, and can also discharge such energy. The rectifying element 104 is made up of a diode that allows current to flow in only the forward direction. The cathode side of the diode is connected between the DC power supply 102 and the LEDs 101, and the anode side is connected to the coil 103.
One end of the transfer switching element 105 is connected between the coil 103 and the rectifying element 104, and the other end is connected to the ground G. The transfer switching element 105 is configured so as to be capable of on/off switching. When the transfer switching element 105 is on, a first closed circuit 111 is formed by the DC power supply 102, the LEDs 101, and the coil 103, and when the transfer switching element 105 is off, a second closed circuit 112 is formed by the LEDs 101, the coil 103, and the rectifying element 104. The current sensing resistor 106 is disposed in order to sense the current value of the current flowing in the LED drive circuit 100. The control apparatus 107 is configured so as to be able to sense the current flowing in the current sensing resistor 106, and control the on/off state of the transfer switching element 105 based on the sensing.
With the LED drive circuit 100 having such a configuration, first, when the control apparatus 107 is operated, the control apparatus 107 switches on the transfer switching element 105, and thus the first closed circuit 111 is formed by the DC power supply 102, the LEDs 101, and the coil 103. When the first closed circuit 111 is formed, a forward bias is applied to the LEDs 101, and the LEDs 101 emit light. Also, since current is flowing in the LED drive circuit 100, current flows to the coil 103, and energy is stored in the coil 103 from such current.
Subsequently, when the current value of the first closed circuit 111 reaches a predetermined upper limit target value, the control apparatus 107 switches off the transfer switching element 105 based on the current sensed in the current sensing resistor 106. The first closed circuit 111 is therefore cut off, and the second closed circuit 112 is formed by the LEDs 101, the coil 103, and the rectifying element 104. When the second closed circuit 112 is formed, a forward bias is applied to the LEDs 101 using the energy accumulated in the coil 103, and the LEDs 101 emit light. Specifically, although power is no longer supplied by the DC power supply 102 when the second closed circuit 112 is formed and the first closed circuit 111 is cut off, a constant current continues to flow in the LED drive circuit 100 due to the discharge of energy accumulated by the coil 103. Accordingly, the LEDs 101 continue to emit light. At this time, the coil 103 attempts to continue to discharge a constant current to the second closed circuit 112, and therefore counter-electromotive force for the continued discharge of a constant current is generated in the coil 103.
Thereafter, when the current value of the second closed circuit 112 reaches a predetermined lower limit target value, the control apparatus 107 again switches on the transfer switching element 105 based on the current sensed in the current sensing resistor 106, and the first closed circuit 111 is formed. Accordingly, the DC power supply 102 applies a forward bias to the LEDs 101, and the LEDs 101 continue to emit light.